1. Field of the Invention
The present invention generally relates to packaging and more particularly to an apparatus for spiral wrapping a load by a web of stretched plastic film by the relative rotation of said load and a film dispenser roller.
2. Prior Art
Numerous conventional wrapping machines develop the forces needed for pulling the web of wrapping film off the roller on which said web of film is supplied, for stretching the stretchable film and for the transportation of said web of film from said film supply roller to the load by the relative rotation of the load to be wrapped and the film dispenser roller, said forces being transmitted along the film web. In such wrapping processes, the tensile load to which the web of film is exposed should be as small as possible from the beginning of the wrapping operation until the completion of at least part of the first relative rotation, the first part of the film web being therefore wound loosely and without elongation onto the load (see West German Pat. No. 24,13,807). The reduction in film tension at the beginning of the wrapping operation is particularly important in the case of a stretch wrapper device provided with two spaced prestretch rollers interconnected by gearing to operate at two different peripheral speeds having a constant relationship to each other to elongate the film web at ambient temperature by 80% or even more.
U.S. Pat. No. 4,497,159 (Lancaster, III) divulges an apparatus for applying stretched plastic film to loads using a prestretching mechanism in the form of two frictionally engaged rollers driven by the film web at different speeds to elongate the plastic film between the engaged rollers beyond its yield strength. An initial portion of the wrap is placed on the load in a substantially unstretched condition, the rollers being forced apart at the beginning of the wrapping cycle. To force the rollers apart, a cam assembly is mounted on a carriage which may be driven upward and downward, keeping the moveable prestretch roller and its associated frictionally engageable wheel disengaged until said carriage has travelled a certain distance at the beginning of each new wrapping cycle. The cam assembly known from said patent implies certain disadvantages, as it necessitates the use of a bottom plate below the carriage limiting the downward travel of said carriage and interfering with low wrapping operations. The floor clearance of state-of-the-art pallet stretch wrappers such as the machine marketed by the applicant under the trademark "Robopac" travelling around the pallet to be wrapped is reduced by said bottom plate, said reduction in floor clearance being particularly cumbersome if the floor surface is irregular or uneven.